There is a wireless power transfer system that wirelessly transfers power between coils, using the coupling coefficient or mutual inductance between the coils. When power is transferred by resonating the resistance and conductance of the coils, a higher maximum value of transfer efficiency between the coils is obtained as a product of the coupling coefficient k between the coils and the Q value of the coils becomes higher.
To increase the transfer efficiency between the coils, there is also known a technique of performing control such that the coils are operated to detect the positions of the coils, by which the distance between the coils is minimized.
The value of coupling coefficient between the coils where wireless transfer is performed varies depending on the shape of the coils, etc., even at the same distance between the coils. If the coupling coefficient between the coils changes significantly, then even if the same power is to be transmitted, there is a need to change the voltage on the power transmitting side or change the voltage conversion ration ratio of the DC-DC converter on the receiving side.